Can't help you with working line breeders. What I would do if I had the time, and was looking for a dog for a specific purpose, is I would train the dog I had, in that venue. Who cares if the dog isn't a cracker jack. Just go at your and your dog's pace, and keep on going.
And while you are there, start noticing other dogs you like. Not their looks, but how they learn, how they act, etc. Start asking the people whose dogs you like, where they got their dog.
I think that it may come to pass that breeders will be wanting more than ever to deal locally. And won't be shipping puppies as much, so take your time, get known in the training circles, and start paying attention to who is breeding the dogs in your neighborhood. Go home and write it down in a log book.
Understand that not every out of control or even reactive puppy out there is caused by the genetics. But as you continue to log the shepherds that you meet and like or dislike, especially notice patterns you are seeing.
Get to know the trainers and ask for recommendations. But, do not just accept a recommendation. There really aren't any shortcuts. You can narrow the field some, but you still need to get out there and see the dogs. And the best way to do this is while training the dog you have. And 2-3 years down the line, you not only go to a great place to get a puppy, but because of your continued training, you avoid a lot of the pitfalls of raising a youngster.
Jenna, RN CGC & Babs, CD RA CGC HIC
Heidi, RA CGC
SG3 Odessa, SchH1, Kkl1, AD
Ninja, RN CGC & Milla, RN CGC
Joy, Star Puppy, RN CGC
Dolly CGC & Bear CGC